1. Field of the Invention
The present invention relates to electrical protective filters or transient voltage surge suppressors (TVSS) for office equipment or any microprocessor-based equipment. These suppressors provide surge filtration to protect the office equipment from surges or transients in the power furnished from the power distribution network.
2. Description of the Prior Art
Numerous types of office equipment used in recent years contain electronic chips or components which are sensitive to voltage surges or transients in the power distribution network provided by utility companies. Examples of such types of office equipment include computers, facsimile machines, photocopiers and the like. Devices known as power filters or transient voltage surge suppressors have been developed to protect these types of office equipment from such power surges or transients. U.S. Pat. No. 5,721,661, of which applicant is inventor, is one form of such a power filter.
Office buildings and other places where such equipment has been used have often been based on a three-phase or center-tap system of power distribution. There are several advantages from the use of this form of power. It allows more power to be delivered to a site, is more easily distributed, and allows two voltage levels (such as 120 volts and 240 volts) to be delivered. Customers can thus use equipment requiring larger amounts of power. These forms of power are a more cost effective way for an electric utility to transmit power to an end user. In three phase or center tap power, in the event of loss of the ground wire or neutral wire connection in the distribution grid, voltage levels provided in the network could increase up to a double level. This is not an uncommon event. Equipment rated at 120 volts could thus be subjected to up to 240 volts. So it could possibly bum out a surge protection component and possibly cause firing the TVSS unit, which could also bum out the connected equipment.
So far as is known, it was typical to use relatively inexpensive thermal fuses for protective purposes in these situations. These fuses would typically open after response to excess heat for periods of from a couple of seconds to several minutes. It may protect the TVSS against firing after bum out, but sensitive electronic chips and circuits of connected equipment were not capable of withstanding such excessive voltage levels for even short fractions of a second, such as a few milliseconds.